Post-outburst radio observation of the region around McNeil's nebula (V1647 Orionis)

We present post-outburst ( 100 days after outburst) radio continuum observation of the region (∼ 30'× 30') around McNeil's nebula (V1647 Orionis). The observations were carried out using the Giant Metrewave Radio Telescope (GMRT), India, at 1272 MHz on 2004 Feb. 14.5 UT. Although 8 sources have been detected within a circular diameter of 25' centred on V1647 Ori, we did not detect any radio continuum emission from McNeil's nebula. We assign a 5σ upper limit of 0.15 mJy/beam for V1647 Ori where the beam size is 5.6''×2.7''. Even at higher frequencies of 4.9 and 8.5 GHz (VLA archival data), no radio emission has been detected from this region. Three scenarios, namely emission from a homogeneous HII region, ionised stellar wind and shock-ionised gas, are explored in the light of our GMRT upper-limit. For the case of a homogeneous HII region, the radius of the emitting region is constrained to be ⪉ 26 AU corresponding to a temperature ⪆ 2500 K, which is consistent with the reported radio and Hα emission. In the ionised stellar wind picture, our upper limit of radio emission translates to dot{M}/v_∞< 1.2{-}1.8×10^-10 M_⊙ yr^-1 km^-1 s. On the other hand, if the stellar wind shocks the dense neutral (molecular) cloud, the radio upper limit implies that the fraction of the wind encountering the dense obstacle is <50%. Based on a recent measurement of X-ray outburst and later monitoring, the expected radio emission has been estimated. Using our radio limit, the radius (⪉36 AU) and electron density (⪆ 7.2×10⁷ cm^-3) of the radio emitting plasma have been constrained using a two phase medium in pressure equilibrium for a volume filling factor of 0.9.